1. Field of the Invention
The present invention relates to an active-matrix display device using thin film transistors as pixel-driving switching devices and a method for driving the display devices, and in particular, to a technique for improving image quality by eliminating crosstalk (hereinafter referred to as xe2x80x9cvertical crosstalkxe2x80x9d if necessary) appearing in the vertical direction of a screen.
2. Description of the Related Art
The general structure of an active-matrix display device will be described with reference to FIG. 11. FIG. 11 consists of circuit diagrams showing two pixels extracted from the conventional active-matrix device. The active-matrix display device includes rows of gate lines X, columns of signal lines Y, and a matrix of liquid crystal pixels LC arranged in the region where the rows and the columns intersect. There are also formed thin film transistors Tr as switching devices for driving the pixels LC. The gate electrodes G of the thin film transistors Tr are connected to the corresponding gate lines X, and either the source electrodes S or the drain electrodes thereof are connected to the corresponding signal lines Y, with the other electrodes connected to the corresponding liquid-crystal pixels LC. In general, the pixels LC are driven by an alternating current. Thus, the polarity of a video signal to be written in each liquid-crystal pixel LC is inverted. Each drain electrode D and each source electrode S are alternately switched in accordance with this polarity inversion. Here, an electrode (H) having a high voltage is called a xe2x80x9cdrain electrodexe2x80x9d, and an electrode (L) having a low voltage is called a xe2x80x9csource electrode Sxe2x80x9d. A vertical scanning circuit (not shown) is connected to each gate line X. The vertical scanning circuit sequentially scans the gate lines X during one vertical period (1F), and selects one row of pixels LC every horizontal period (1H). In addition, a horizontal scanning circuit (not shown) is connected to each signal line Y. The horizontal scanning circuit samples a video signal Vsig for each signal line Y, and writes the video signal Vsig in the one row of pixels selected in one horizontal period.
The active-matrix display device has an inferior condition called vertical crosstalk. Thus, when the active-matrix display device is used in an apparatus such as a projector, generated image quality deteriorates, which is a problem to solve. As shown in FIG. 11, the vertical crosstalk is caused by the asymmetry of currents leaking from the thin film transistors Tr. In the condition shown in FIG. 11, left, the signal line Y is at level L, with the H-level signal written in the pixel LC. In this condition, a leakage current flowing when the gate electrode of the thin film transistor Tr is cut off is represented by Ioff1. In addition, in FIG. 11, right, the thin film transistor LC is maintained at level L, the H-level signal is applied to the signal line Y. In this condition, a leakage current flowing when the gate electrode of the thin film transistor Tr is cut off is represented by Ioff2. In general, Ioff1 is larger than Ioff2 because of the asymmetry of the thin film transistors Tr.
For example, as shown in FIG. 12, displaying a black window 30 in the center of a screen 20 generates vertical crosstalk in portions A, and the brightness of the portions A differs from normal portions B. A video signal Vsig to be written into each pixel LC is expressed by VsigCxc2x1xcex94V where VsigCxc2x1xcex94V represents a center potential, e.g., 6 volts; the symbol xc2x1 means that the video signal Vsig is inverted every horizontal period; and xcex94V represents a change of Vsig in reference to VsigC. When the maximum change is represented by xcex94V(MAX), xcex94V(MAX) is, e.g., 4 volts. In normally white mode, VsigCxc2x1xcex94V(MAX) (=6xc2x14 volts) is written in the black window 30.
Thus, a voltage of 10 or 2 volts is applied to the liquid-crystal pixels LC included in the black window 30. In addition, an intermediate-level video signal of 6xc2x12 volts is written in the liquid-crystal pixels LC included in the background of the screen 20 excluding the black window 30. Accordingly, the background is grey, and a voltage of 8 or 4 volts is applied to each pixel LC.
FIG. 13 shows that the potentials of the pixels LC included in the portions A and B shown in FIG. 12 change during two vertical period (2F). During the change, the operating condition of the corresponding thin film transistors Tr chronologically changes. The periods of the change are represented by T1 to T4. The operating condition of the thin film transistors Tr corresponding to the pixels LC included in the portions A changes as shown in periods T1, T2 and T1 in the initial one vertical period (1F), and changes as shown in periods T3, T4 and T3 in the subsequent one vertical period. The operating condition of the thin film transistors Tr corresponding to the pixels LC included in the portions B changes as shown in period T1 in the initial one vertical period (1F), and changes as shown in period T3 in the subsequent one vertical period.
FIG. 14 schematically shows the operating conditions of each thin film transistor Tr in periods T1 to T4. In period T1, a voltage of 8 volts is applied to the corresponding pixel LC, and the potential of the signal line Y oscillates between 8 and 4 volts every horizontal period. The leakage current at this time flows in the direction of Ioff1. In addition, in period T3, the pixel is at 4 volts, and the potential of the signal line Y oscillates between 4 and 8 volts. The leakage current flowing at this time has a polarity identical to that of current Ioff2. The operating condition of the thin film transistors Tr included in portions B is alternately repeated between periods T1 and T3 every vertical period (1F). The pixel potential caused by the leakage current changes as represented by a dotted line shown in FIG. 13. The operating condition of the thin film transistors included in portions A is basically similar. However, a video signal of 2 or 10 volts is written in the pixels included in the window 30 during periods T2 or T4, which oscillates the signal line Y between 10 and 2 volts within the writing period. For example, during period T2, a voltage of 8 volts is applied to the pixels LC, which changes the potential of the signal line Y between 10 and 2 volts. The amounts of the leakage currents in periods T1 and T2 differ due to the asymmetry of the leakage currents. Accordingly, as shown in FIG. 13, the pixel potential slightly differs in portions A and B in period T2, which causes the vertical crosstalk. Similarly, in period T4, the potential of the pixel is maintained at 4 volts, while the potential of the signal line Y oscillates between 10 and 2 volts every horizontal period (1H). The leakage currents in the thin film transistors Tr differ in periods T3 and T4, which generates the difference in the pixel potential in portions A and B during period T4. In particular, differently from period T3, period T4 includes a condition where the signal line Y is at level L of 2 volts. Thus, the leakage current flowing increases, which causes portions A and B to have an extremely remarkable potential difference.
In addition, the active-matrix display device has a problem of having not only the above-described vertical crosstalk but also vertical fixed-pattern noise, which will be described by referring to FIG. 4. An example of the active-matrix display device includes rows of gate lines X and columns of signal lines Y, a matrix of pixels LC arranged in the region where the gate lines X and the signal lines Y intersect, and thin film transistors Tr for driving the pixels LC. The active-matrix display device includes a vertical scanner 1 which sequentially scans each gate line X, and selects one row of pixels LC every horizontal period. The active-matrix display device includes a horizontal scanning circuit 4 which samples video signal Vsig for each signal line Y, and writes video signal Vsig in one row of pixels LC selected every horizontal period. This horizontal scanning circuit 4 consists of horizontal switches HSW provided at ends of the respective signal lines Y, and a horizontal scanner 4 for sequentially switching the horizontal switches HSW. The signal lines Y are connected to a video line 2 via the horizontal switches HSW. This video line 2 is supplied with video signal Vsig from a signal driver 3. The horizontal scanner 4 outputs sampling pulses xcfx86H1, xcfx86H2, and xcfx86H3 to xcfx86HN.
FIG. 5 shows the waveforms of sampling pulses xcfx861H, xcfx86H2 and xcfx86H3 sequentially output from the horizontal scanner 4 shown in FIG. 4. As the number of pixels increases in accordance with high integration of an active-matrix display device, a video-signal sampling rate accelerates. As a result, there appears a change in the width xcfx84H of each sampling pulse. When each sampling pulse is applied to the corresponding horizontal switch HSW, video signal Vsig supplied from the video line 2 is sampled for each signal line Y via the horizontal switch HSW in conduction. Since each signal line Y has a predetermined capacitance component, each signal line Y charges or discharges in accordance with each sampling pulse. This causes the video line 2 to obtain potential. As described above, increasing the sampling rate causes each sampling pulse to have a different width. Thus, the amount of charging or discharging is not constant, and the potential of the video line 2 changes. Since this potential change is superimposed on video signal Vsig, vertical fixed-pattern noise is generated in a displayed image, so that the image quality deteriorates disadvantageously.
In order to solve this problem, there is a proposed precharge method, which is disclosed in, for example, Japanese Unexamined Patent Publication No. 7-295521 which was filed by the assignee of the present application. In FIG. 6 is shown a precharge active-matrix display device. This precharge active-matrix display device is basically similar to the active-matrix display device according to the present invention. Accordingly, components corresponding to those in FIG. 1 are denoted by the corresponding reference numerals for facile understanding. As shown in FIG. 6, the precharge active-matrix display device includes a precharge means 5a which supplies predetermined voltage signal (precharge signal) Psig to each signal line Y just before video signal Vsig is written in one row of liquid-crystal pixels, and which reduces the amount of charging or discharging by each signal line Y. In this case, the precharge means 5 includes a plurality of switches PSW connected to ends of the signal lines Y, and a control means 6a for applying precharge signal Psig to the signal lines Y by simultaneously switching the switches PSW. This control means 6a simultaneously switches the switches PSW by outputting control pulses PC. Precharge signal Psig is supplied from a signal source 7a provided separately from a signal drive 3. This precharge signal Psig has a grey level (intermediate level), differently from video signal Vsig changing between while level and black level.
The operation of the active-matrix display device shown in FIG. 6 will be described below by referring to a timing chart shown in FIG. 7.
Vertical clock signal VCK input to the vertical scanner 1 has a pulse width corresponding to one horizontal period. Control pulses PC output from the control means 6a are output within a horizontal non-effective period such as a horizontal blanking period. Horizontal start pulses HST supplied to the horizontal scanner 4 are output every horizontal period, just after control pulses PC are output, which start the sampling of video signal Vsig. The sampling of video signal Vsig is successively performed synchronizing with horizontal clock signal HCK supplied to the horizontal scanner 4. In addition, since the polarity of video signal Vsig supplied from the signal driver 3 via the video line 2 is inverted every horizontal period, ac driving is performed. In accordance with this polarity inversion, the polarity of precharge signal Psig supplied from the signal source 7a is also inverted every horizontal period so as to coincide with the polarity of video signal Vsig. Precharge signal Psig has a potential level Vp with reference to the center potential VsigC of video signal Vsig, and represents the grey level positioned between the white level and the black level. The potential level of precharge signal Psig is basically set at the grey level (intermediate level) whose uniformity is most easily recognized in visual characteristics. The bottom waveform in the timing chart represents a change in potential VY applied to each signal line Y. When control signal PC is output at the start of one horizontal period and the switches PSW are in conduction, precharge signal Psig is applied to all the signal lines Y so that their capacitance components can charge or discharge. The application of precharge signal Psig changes the potential of each signal line Y to level Vp. Subsequently, actual video signal Vsig is sampled for each signal line Y, and the potential of the signal Y changes in accordance with video signal Vsig to perform writing. Potential change xcex94v caused by writing decreases to Vsigxe2x88x92Vp, which reduces the amount of charging or discharging. This enables control of a shift in the potential of the video line 2, which remarkably improves uniformity. In the above-described precharge method, all the signal lines Y are precharged up to intermediate-level potential at timing with no influence on a display image, such as a horizontal blanking period, signal-line charging or discharging current generated when actual video signal Vsig is sampled is reduced to control a shift in the potential of the video line 2. In other words, the switches PSW are used to finish charging or discharging each signal line Y in the blanking interval, and charging or discharging current caused by the actual video signal Vsig is generated by the difference in potential level between precharge signal Psig and video signal Vsig.
The level setting of precharge signal Psig has a problem to solve, which is shown in FIG. 8. The closer to the level of video signal Vsig, the more preferable the level of precharge signal Psig. In particular, when the level of precharge signal Psig is fixed at a predetermined level, it is preferable to set the level of precharge signal Psig at grey level which remarkably generates vertical fixed-pattern noise. In FIG. 8, the grey level is represented by dotted lines PsigH2 and PsigL2. The setting at the grey level generates vertical crosstalk. Accordingly, it is preferable to increase the amplitude of precharge signal Psig. The amplitude is represented by PsigH1 and PsigL1. In particular, by setting amplitude PsigL1, to the minimum level of video signal Vsig or less in a period during which video signal Vsig is at low level, the vertical crosstalk can be remarkably controlled. Therefore, the vertical fixed-pattern noise is generated when setting the level of voltage signal (precharge signal) Psig to a voltage (PsigH2 or PsigL2) at which the vertical fixed-pattern noise least appear or when setting the level of voltage signal (precharge signal) Psig to a voltage (PsigH1, or PsigL1) at which the vertical crosstalk does not appear.
Accordingly, it is an object of the present invention to provide an active-matrix display device and a method for driving the display device in which image quality is improved by eliminating vertical crosstalk and fixed-pattern noise.
To this end, according to an aspect of the present invention, the foregoing object has been achieved through provision of an active-matrix display device including: a plurality of rows of gate lines; a plurality of columns of signal lines; a matrix of pixels provided in the region where the gate lines and the signal lines intersect; a vertical scanning circuit for sequentially scanning the gate lines in one vertical period, and selecting one row of pixels every horizontal period; a horizontal scanning circuit for sampling a video signal for each signal line before writing the video signal in the selected one row of pixels; and a voltage applying circuit for applying to each signal line a voltage equal to or less than the minimum level of the video signal in one horizontal period excluding a time assigned for writing the video signal in one row of pixels.
Preferably, the voltage applying circuit repeatedly adjusts signal leakages from all the pixels to an almost equal value during one vertical period.
The voltage applying circuit may comprise a circuit for precharging each signal line by changing a voltage equal to or less than the minimum level of the video signal to the intermediate level of the video signal and applying the changed voltage to each signal line after applying the voltage equal to or less than the minimum level of the video signal to each signal line before the horizontal scanning circuit writes the video signal in each signal line.
The horizontal scanning circuit may write the video signal, whose polarity is inverted every horizontal period, and the voltage applying circuit may apply to each signal line a voltage equal to or less than the minimum level of the video signal having either polarity in a horizontal period during which the video signal having either polarity is written.
According to another aspect of the present invention, the foregoing object has been achieved through provision of an active-matrix driving method for driving an active-matrix display device including a plurality of rows of gate lines, a plurality of columns of signal lines, and a matrix of pixels provided in the region where the gate lines and the signal lines intersect, in which the active-matrix driving method comprises the steps of: vertical scanning for sequentially scanning the gate lines during one vertical period, and selecting one row of pixels every horizontal period; horizontal scanning for sampling a video signal for each signal line before writing the video signal in the selected one row of pixels; and applying a voltage equal to or less than the minimum level of the video signal in one horizontal period excluding a time assigned for writing the video signal in one row of pixels, and repeatedly performing the voltage application to adjust signal leakages from all the pixels to an almost equal value.
Preferably, after the voltage equal to or less than the minimum level of the video signal is applied and before the video signal is written, the voltage is changed to the intermediate level of the video signal and the changed voltage is used to charge each signal line.
The video signal, whose polarity is inverted every horizontal period, may be written, and in a horizontal period during which the video signal having either polarity is written, the voltage equal to or less than the minimum level of the video signal may be applied to each signal line.
According to the active-matrix display device, when it is applied to, e.g., a projector, intense light from a light source is incident on a panel to generate vertical crosstalk. This vertical crosstalk is caused by the asymmetry of leakage current from thin film transistors. Therefore, according to the present invention, a voltage equal to or less than a video signal is input to all signal lines so that signal leakages from all pixels can be approximately equalized, which prevents the vertical crosstalk from occurring.